So there's been several times that folks have posted quotes from one master or another on "keeping some of your breath" in Uechi.
In the hard form of the jar training I try to build up a good bit of internal pressure without unsafely sending my blood pressure too high. I pressurize my core and the pressure pushing out from the air works against the pressure of the contraction of my muscles against that pressure (again - in the core).
When I'm not doing the jar training I still maintain a level of internal pressure with my breath to give my muscles something to contract against - but not as much as I do with the jar training.
You can really hurt yourself if you try to build up too much internal pressure - so if you've got high blood pressure, a history of TIAs, heart disease, diabetes, or a stroke or a hernia in your past - this isn't something to play with.
It has been called "tiger training" to me by more than one senior person. The internal pressure is one part, the pulling down of shoulders another, the raising of the lats another, and the overall contraction of the core muslces another.
In this process the vascular system, the fascia, the ligaments, the tendons, and the muscles are all addressed and given a certain amount of stress to encourage development.
Like folks always say...sanchin is a full body exercise!